Chris Poole (Moot) founded 4chan after being banned from SA (in the version I heard it was on accusations of pedophilia, because anyone who finds anime characters attractive is clearly a pedophile). SA was far more influential in creating and shaping 4chan than anything else, even the Japanese forums it was imitating (2chan, etc.)
Would there still be something that fits roughly the same niche as 4chan, if 4chan didn't exist? Most likely yes. But it wouldn't be anything like the 4chan we know today.
And, I don't think you can blame SA for 4chan, nor 4chan for trump. Both trump and 4chan meet a demand, and that demand would have been filled, one way or another.